There are a number of applications in which it is desirable to remove contaminants from a fluid such as air. Radon accumulation in homes, schools and other buildings is recognized as a serious health problem. Radon is a radioactive noble gas produced in the ground by the natural decay of uranium and radium. The radon decays to a chain of daughter products which associate with dust particles in the air. The daughter products emit harmful alpha radiation which, when inhaled, damages lung tissue. Tens of thousands of cases of lung cancer are estimated to arise each year from radon infiltration into homes.
Radon adsorption on activated charcoal has been utilized to remove radon from uranium mines. Through a cycling adsorption process radon is collected within a charcoal bed and then removed for disposal. As is true of other cycling adsorption processes commonly used to remove contaminants from a gas stream, the goal of the process is to concentrate the contaminant to facilitate disposal of the contaminant. Consequently, the charcoal bed once contaminated is cleansed by passing gas at a lower rate through the bed to produce a regeneration gas stream which has a higher contaminate concentration than the initial contaminated air. However, the radon concentration of the charcoal bed can be lowered only to the concentration level of the regeneration gas stream which is obtained from the contaminated mine and is usually heavily contaminated. Since the adsorption of radon varies inversely with temperature, the beds are typically heated during regeneration to enhance release of the radon and prevent adsorption of the radon in the regeneration gas. Heating also reduces the flow of regeneration gas required to cleanse the charcoal bed. Dual charcoal beds are operated simultaneously so that one removes contaminants while the other is regenerated. Additional charcoal beds are sometimes provided to further concentrate the regenerative gas stream.
There are several problems with this cycling adsorption technique, particularly when applied to use in homes. A great deal of energy is required to heat the beds during regeneration. During the winter the cost of heating the bed during regeneration would be compounded by the loss of indoor heated air utilized as a regeneration gas to be vented outdoors.